Our invention relates to an orthopedic jig or saw guide for preparing a femoral knee joint to receive the femoral component of a posterior stabilized prosthesis.
The knee is the intermediate joint of the human leg. It is mainly a joint with one degree of freedom, that is, it bends in one plane. However, the knee has an accessory or second degree of freedom, rotation about the long axis of the leg, which occurs only when the knee is flexed. A replacement prosthesis, like the natural knee itself, must attempt to reconcile two mutually exclusive conditions. These are, first, to provide stability in extension, thus supporting the body weight, and, second, to provide mobility when the knee is flexed.
In the healthy knee, these requirements are met by a combination of bone structure and restraining ligaments. Many prosthetic knees attempt to retain these advantages by replacing only the bony structure or cartilage, while retaining the ligaments of the knee. In some cases, however, it is not possible to preserve all of the ligaments or the ligaments have themselves become substantially weakened. One of the known designs to accommodate such conditions is known generally as a posteriorly stabilized knee prosthesis. Such a structure is known, for example, from U.S. Pat. No. 4,213,209 to Install, et al. and U.S. Pat. No. 4,298,992 to Berstein, et al. The fernoral component of a posterior stabilized knee prosthesis has, broadly speaking, two condyles separated by a central box or cavity. Posteriorly there is a transverse cam whose purpose will be explained below.
The tibial component of a posterior stabilized knee prosthesis is characterized by a central post, frequently part of a high density polyethylene articulating surface which is adapted to extend upwardly between the condyles of the femoral component and into the cavity mentioned above. This post, riding against medial and lateral walls of the cavity, gives addition stability to the prosthesis. Moreover, as the knee is bent, the transverse cam will contact the post on a specially designed cam follower surface causing the knee to flex in a more anatomical fashion.
The assignee of our invention has developed a posteriorly stabilized knee prosthesis similar of that described in the '992 patent. The central housing or cavity, however, has a curved roof on the cavity of the femoral component. We have developed a specialized jig for creating the necessary cavity in the distal end of a femur to receive such a posterior stabilized femoral component.